The present invention relates to a brushless DC motor comprising a rotor and a stator having a plurality of magnetic pole teeth, and is particularly concerned with the miniaturization and the high operating efficiency of such a brushless DC motor.
A prior art brushless motor having three teeth is known as disclosed in Japanese Patent publication No. 55-12835(1980). A stator of the motor mentioned in the publication has three stator magnetic poles (teeth) disposed at regular intervals (at every 120 degrees) on a circle concentric with the rotating shaft of a permanent magnet rotor, and users stator core for coupling the stator magnetic poles with a yoke disposed on a circle concentric with the rotating shaft. A field winding is wound on each of the stator magnetic poles. Accordingly, an electric current is fed to the three field windings successively to generate a rotational magnetic field.
In the prior art brushless motor constructed as above, since there is no split provided on the yoke, it is necessary to start winding the field windings from inside the stator, and thus the efficiency of manufacture is deteriorated due to the breakage of the windings easily at the time of this winding.
Further, an increase in the number of the field windings may result naturally in an increase in the number of crossovers among the field windings, thus further reducing the efficiency of manufacture.
In addition the size of the motor increases with an increase in the number of stator magnetic poles, which prevents the miniaturization of the motor.
In view of such problems, an attempt was made to realize a small brushless motor with the number of stator windings reduced from three to two, however, the combination and timing of the current fed to the stator windings became difficult, thus leaving another problem that prevents the enhancement of the thermo operating efficiency.
Meanwhile, a conventional small motor is structured, for example, as disclosed in Japanese Patent Publication No. 59-18940(1984). The induction motor mentioned in the publication is structured such that core parts of the stator core on which a coil is wound are punched separately out of a steel plate, built up in layers, and after the coil is wound on the core parts, the core parts are held between legs of the stator core to form a stator.
When assembling the small motor as above, or more particularly when forcibly fitting the core parts between legs of the stator core, the squareness formed by the legs and the core parts must be carefully controlled, thereby reducing the efficiency of manufacture. The fabrication becomes difficult as the number of interfaces at the time of fitting increases, and thus is extremely difficult, when the number of interfaces is high. In addition, forcible fitting of the core parts while controlling squareness in the structural relationship between the core parts and legs may introduce an unreasonable force, thus entailing (i) a deformation of the core or particularly of pole separating parts, as notches; (ii) degradation of roundness in the bore of the stator; (iii) a separation of the iron plate at a press-fit zone to cause a defective stator core, and thus the characteristics of the motor degenerate, and vibrations occur more severely.